It is known in the field of mobile communications to use PIFA's (planar inverted-F antennas) to achieve a relatively large bandwidth at particular frequencies that may be used for the transmission and reception of mobile communications. Such a prior art PIFA is shown in FIG. 1. FIG. 1A shows a ground plane 1 with a conducting plate 2 mounted above it by means of a short circuit plate 4. FIG. 1B shows a side view of the PIFA and FIG. 1C shows a top view of the conducting plate 2. The frequencies at which a PIFA produces resonances, and the bandwidths of generated frequencies depend on the geometry of the PIFA. Relevant parameters include the length and width of the conducting plate 2 (x and y); the position of the connecting wire 3 (Ix, Iy); the width of the short circuit plate 4 (w); the height of the conducting plate 2 above the ground plane 1 (h); and the radius of the wire 3. These parameters can be adjusted to provide optimal bandwidth at a particular frequency. The plate 2 will typically be a quarter wave structure.
It is also known to use a double PIFA antenna, in which an additional resonator, or parasitic resonator, is positioned between the ground plane and the main resonator and parallel to them. Such a configuration is shown in FIG. 2. Above the ground plane 21 is a parasitic resonator 25, and above that is a main resonator 22. These three components are electrically connected together by means of a short circuit plate 24. In addition, a feed cable 23 passes through a hole in the ground plane 21 and a hole in the parasitic resonator 25, and the inner conductor of the feed cable (“source +”) makes electrical contact with the main resonator 22. The outer conductor (“source −”, or ground) is connected to the ground plane 21. This arrangement provides a greater bandwidth than is achievable with a single PIFA. In addition, the parasitic resonator can provide an extra resonance. The parasitic resonator is excited indirectly by the main resonator, rather than directly by the feeding cable. This geometry (FIG. 2) is a variation on the well known ‘PIFA with parasitic element’ design, where the parasitic element is placed largely in the same plane and adjacent to the main resonator. In the structure of FIG. 2, the main resonator 22 and the parasitic resonator 25 are both quarter wave plates.
The frequency bands used in GSM mobile communication systems are currently USGSM850, EGSM900, DCS1800, and PCS1900. USGSM is a frequency band commonly used in North America; EGSM is used in Europe and ranges from around 880 to 960 MHz; DCS1800 is a common “digital cellular service” band ranging from 1710 to 1880 MHz; and PCS1900 is a common “personal communications service” frequency band. Mobile telephones that are capable of transmitting and receiving signals at all of these frequency bands are known as “quad-band”. It is known in the field of mobile communications to use a slotted PIFA pair antenna (see U.S. Pat. No. 6,621,455 entitled “Multi-band Antenna”) to achieve resonance at frequencies within the four GSM bands.
A slotted PIFA pair antenna is shown in FIG. 3A. This configuration behaves as two adjacent PIFA's. A printed circuit board 31 acts as a ground plane of the antenna. A planar conductive layer 32B, comprised of copper or any other highly conductive material with slots 35 etched out it, forms a radiating element. The geometry of this layer dictates the resonant frequencies obtainable by the antenna and their bandwidths. The conductive layer 32B is typically supported by an insulating substrate 32A.
Conducting pins 30 are used to ground the conductive layer 32B, and a feed is provided on the underside of the supporting substrate 32A. FIG. 3B shows a detailed view of the feeding structure of this slotted PIFA pair. Typically, a coaxial cable would be used to feed the antenna. The inner and outer conductors of the cable would be connected to different points on the antenna structure. In FIG. 3B, 34A is a conductive strip connected to “source +” and this strip is positioned along the centre of the substrate 32A, preferably extending parallel to the slot 35. 34B is connected to “source −” and protrudes from the printed circuit board 31. There is a small gap between the feed strips 34A and 34B. The coaxial cable connects the antenna to transmitter and receiver circuitry.
It has been found that such a slotted PIFA pair antenna is less effective for telecommunications handsets whose covers are made substantially of metal. In the case of metal mobile terminals, the bandwidth achievable by a slotted PIFA pair antenna such as that shown in FIGS. 3A and 3B is significantly worse compared with non-metal terminals at the lower GSM band (USGSM850 and EGSM900). 30-40% bandwidth reduction is experienced with metal handsets due to the large metal blocks used therein. However, metal handsets have an appearance of quality and luxury and are aesthetically appealing. It is therefore desirable to produce a quad-band mobile handset that may be made of metal.